A tire is installed on a vehicle wheel to provide traction between the vehicle and the road as well as a flexible cushion to absorb shock. Most tires are pneumatic inflatable structures, comprising a doughnut-shaped body of cords and wires encased in rubber and generally filled with compressed air to form an inflatable cushion.
Due to a tire's construction, a tire may act as a spring and store potential energy when the wheel is turned. This spring-like effect is caused by the friction of the road surface which resists turning of the portion of the tire adjacent a contact patch of the tire with the road. As the wheel is turned, portions of the tire near the wheel turn with the wheel while portions of the tire near the contact patch with the ground may resist the turning motion. As a result, a portion of the tire between the wheel and the contact patch may elastically deform. The elastic deformation of the tire, or more specifically the desire of the tire to return to a non-deformed shape, provides the potential energy referred to as tire windup.
Tire windup may be more pronounced when a vehicle is not moving, or moving slowly, such as when the vehicle is parking. As a result, when the vehicle comes to a rest and the steering wheel is released, the windup of the tire may cause the steering wheel angle to snap to a different angle than the final location. In the case of an autopark event, where an electronic power assist motor turns the steerable wheels, when the autopark event is complete and power is removed from the motor, the tire windup may snap the steerable wheels and/or steering wheel to a different angle then the final desired location, or may leave the steering system locked in a position under load.
The tire wind-up may vary because of the friction between the tire and the road varies, so providing a constant overshoot to allow for tire windup may not allow the steering wheel to rest into a final desired position or remove load from the locked in place system for every scenario.
An Active Park Assist system is capable of performing an autopark event, and may command a final road wheel or steering wheel angle when the system completes a parking event. The system may command a zero angle, but after the vehicle is turned off, the system may relax and the tire windup may move the road wheel or steering wheel away from the zero angle. If the steering wheel is locked in the zero position, then the lock mechanism may be placed under load. A loaded steering wheel lock may cause issues when attempting to restart a vehicle, or may become more difficult to unlock. As well, it may be desirable to leave the steering wheel in a specific angular position other than the zero angle, and any movement from the specific final angular position would not be viewed as desirable.